Retroreflective articles are used extensively for making highway signs, street signs, pavement markings, and the like. In particular, retroreflective articles comprising an array of cube corner retroreflective elements are now well known in the art. Cube corner retroreflective elements are prism structures which have three mutually perpendicular faces meeting at a single corner, such as that configuration defined by the corner of a room or a cube. Cube corner elements operate according to the principle of total internal reflection. That is, a cube corner element receives a ray of incident light from a source and sends it back toward the same source in a direction substantially parallel to the ray of incident light.
Rigid retroreflective articles have been formed from materials including polymethylmethacrylate, polycarbonates, polyethylene terephthalate, and the like. Because such articles are rigid, however, they are difficult to apply to irregular surfaces, e.g., curved surfaces or surfaces with corrugations or rivets. Flexible retroreflective articles would be desirable for imparting retroreflectivity to such irregular surfaces. For example, flexible retroreflective articles could be advantageously used for applications such as vehicle conspicuity, boat and shipping conspicuity, roll-up sign products, personal safety products, internal reflectance for energy control, and the like. Desirable properties for such flexible retroreflective articles would include not only flexibility, but also excellent resistance to weathering; dimensional stability in hot and cold temperatures; flexibility over a wide range of temperatures; and good optical clarity relative to the visible light wavelengths.
Flexible retroreflective articles have been formed with materials such as plasticized polyvinyl chloride. These materials, however, show poor dimensional stability at elevated temperatures. Additionally, the plasticizer used in these materials tends to migrate to the surface, causing discoloration and impairment of retroreflective efficiency.
Relatively thin retroreflective articles having some degree of flexibility have also been formed using thin caliper, high modulus resins such as polycarbonate, polyester, or acrylic resins. Such articles can be coated with an aluminum vapor coat. These articles, however, tend to wrinkle when applied to surfaces having rivets or corrugations.
Attempts have also been made to prepare flexible, dimensionally stable retroreflective articles based on polyurethane polymers. For example, U.S. Pat. No. 4,576,850 discloses a retroreflective article based on a radiation curable aliphatic urethane oligomer. These articles, however, show low tear strength.
U.S. Pat. Nos. 4,582,885 and 4,668,558 disclose retroreflective articles based on acrylate end-capped siloxane urethane oligomers. These articles, too, show low tear strength.
An aliphatic polyurethane obtained by reacting 1,4-cyclohexane diisocyanate, a polydiol, and butane diol has been described, for example, in Gorman et al., Rubber and Plastic News, p. 32 (Apr. 26, 1986); Syed, Rubber and Plastic News, pp. 21-22 (Mar. 21, 1988); and Dieter et al., Polymer Engineering and Science, Vol. 27, No. 9, pp. 673-83 (Mid-May 1987).